In the manufacture of blow molded plastic articles of irregular cross-section, the attainment of proper material distribution throughout the walls has always been a problem. In such irregular shapes, the outermost portions of the article, for example, the corners of a rectangular bottle, are formed from plastic which is "blown-up" to a greater extent than are the side panels of the bottle. As a result, the corners of such bottles are thinner and weaker.
This problem also exists in oval bottles, in bottles having handle-receiving protuberances, and the like.
Various solutions for this problem have been proposed in the prior art, including the extrusion or injection molding of blowable parisons of non-tubular shape conforming to the shape of the container to be finally formed. However, these solutions have been costly and complicated, since the formation of a parison of varied wall thickness, of oval configuration, and the like, requires complicated, matched tooling, constant maintenance, skilled set-up and the like. Further, the thicker portions of a differential thickness parison do not blow up at the same rate as the center parison portions, and the blowing of the various parison portions to the exact, corresponding, desired portion of the final container is difficult and unpredictable.
U.S. Pat. No. 2,336,822 to Wadman discloses a method and apparatus for forming hollow glassware by chilling portions of a glass parison prior to formation of the final article. This results in a "pre-form" which is chilled and expanded in the open air to form cooler peripheral portions which are stretched to the convex corners portions of the blow mold. Thus, in Wadman, that portion of the pre-form which is stretched the most is the coldest.